This invention relates generally to rocket propulsion systems and, more particularly, to electric rocket propulsion systems.
Electric rocket propulsion systems are useful for a variety of space missions because of their high specific impulse I.sub.sp when compared to conventional chemical propulsion systems. Electric propulsion systems produce thrust by converting electrical energy directly into propellant kinetic energy, without necessarily raising the temperature of the propellant. In contrast, chemical propulsion systems produce thrust by converting chemical energy into heat, which is then converted into propellant kinetic energy by an expansion nozzle. As a result, the specific impulse I.sub.sp developed by chemical propulsion systems is limited by the temperature limitations of the nozzle and chamber wall materials, while the specific impulse developed by electrical propulsion systems is not. This allows electrical propulsion systems to develop a higher specific impulse than chemical propulsion systems.
However, electric propulsion systems have relatively low thrust-to-mass ratios and produce very low acceleration levels. Therefore, these systems cannot perform the initial launch function, but once launched into space by chemical propulsion systems, they can provide low acceleration levels continuously over long periods of time. Consequently, these systems are ideal for propelling spacecraft over very long distances or as thrusters for controlling and maneuvering spacecraft over extended periods of time.
Electric rocket propulsion systems employ either electrostatic or electromagnetic forces for generating thrust. Electrostatic propulsion systems generate thrust by ionizing a neutral propellant to form an ion source. The ions are then accelerated using electrostatic forces to produce a high velocity ion beam. An electromagnetic propulsion system generates thrust by ionizing a neutral propellant to form a plasma. Currents induced in the plasma interact with electromagnetic forces through a body or Lorentz force to accelerate the plasma. A third type of propulsion system, although not truly an electric propulsion system, is an electrothermal propulsion system, commonly referred to as an arcjet or plasma jet. An electrothermal propulsion system uses electrical power to heat a propellant to very high temperatures, and then accelerates the heated propellant in a conventional expansion nozzle.
However, none of these propulsion systems operates efficiently in the specific impulse I.sub.sp range of 800 to 2500 seconds, which is the preferred range for earth-orbit missions. Chemical propulsion systems are typically limited to a specific impulse I.sub.sp below this range and electrostatic propulsion systems are usually limited to a specific impulse I.sub.sp above this range. Electromagnetic and electrothermal propulsion systems operate in this range, but they are typically inefficient and require very high electrical power levels. Accordingly, there is a need for an efficient propulsion system that operates in the specific impulse I.sub.sp range of 800 to 2500 seconds. The present invention clearly fulfills this need.